1. Field of the Invention
The invention relates to a method and device for reducing a magnetic unidirectional flux component in the core of a transformer, comprising at least one compensation winding magnetically coupled to the core of the transformer, at least one switching unit arranged electrically in series with the compensation winding in a current path, in order to feed a current into the compensation winding, the effect of which is directed opposite to the unidirectional flux component, and at least one current-limiting reactor arranged electrically in series with the compensation winding in a current path.
2. Description of the Related Art
In electrical transformers, such as those used in power distribution networks, a direct current may undesirably feed into the primary winding or secondary winding. Such a direct current feed, also referred to below as a DC component, may originate from electronic structural components, such as those currently used in the activation of electrical drives or also in reactive power compensation. Another cause may be what are known as ‘geomagnetically induced currents’ (GIC).
In the core of the transformer, a DC component causes a unidirectional flux component that is superimposed onto the alternating flux. This results in a non-symmetrical modulation of the magnetic material in the core and entails a series of disadvantages. A direct current of as little as a few amperes can produce a local temperature rise in the transformer, which can negatively affect the service life of the winding insulation. A further undesirable effect is a raised noise emission during operation of the transformer, because modern transformer cores have a very high magnetic conductivity, so that even low electrical direct currents are sufficient to bring the transformer core, in a half-period of the alternating current, into saturation.
Various active and passive apparatuses are known to reduce the operating noise of a transformer. According to WO 2012/041368 A1, an electrical voltage induced in the compensation winding is used and employed to compensate for the interfering magnetic unidirectional flux component. In this way, a compensation current is generated via an electronic switching unit, where the switching-on of the switching unit follows a predetermined switching strategy, such as via a phase angle control. Here, a thyristor switch is connected in series with a current-limiting reactor to introduce the compensation current into the compensation winding.
By virtue of the cited measures, the thermal load of the winding of the transformer and the losses and noises are lower. Here, the device for reducing a magnetic unidirectional flux component can be realized with comparably simple means using discrete and/or programmable modules. An energy store, such as a battery or a capacitor, is not needed to generate the compensation current; the energy for generating the compensation current is taken directly from the compensation winding. By virtue of its simplicity, the reliability of the switching arrangement is high and well suited to the low-maintenance long-term operation of a transformer in a power distribution network. The field of application comprises both transformers in the low- or medium-voltage range, as well as transformers with a very high power (power transformers, high voltage direct current (HVDC) transformers). Neither the size nor safety-relevant apparatuses or other design criteria of the transformer are disadvantageously influenced by the use of the device.
It is advantageous, in a device for reducing a magnetic unidirectional flux component, for the current-limiting reactor to be installed within the tank of the transformer, in order to use its (oil) cooling. However the current-limiting reactor requires space in the tank of the transformer. Since what is known as a thyristor circuit (consisting of a switching unit embodied as a thyristor and a current-limiting reactor) is often not sufficient to introduce the necessary ampere-turns (i.e., the necessary magnetic force permeation) into the transformer, a number of thyristor circuits are cascaded in a known manner, whereby a number of current-limiting reactors are however also required, as shown in FIG. 2 for instance. Sometimes, in practice, up to four thyristor circuits have to be cascaded, which also means installing four current-limiting reactors into the tank of the transformer. However, a number of current-limiting reactors result in an increased space requirement in the tank of the transformer.